DRY-CHARGED BATTERY WITH A CONTAINER OF A POLYOLEFIN MATERIAL

Abstract: The invention relates to a dry-charged battery having a battery container of polyolefin, and is characterised in that the battery is provided with a surrounding bag or a surrounding casing of a material having a higher oxygen diffusion resistance than the polyolefin material of the battery container, whereby the battery exhibits improved storage properties. The invention also relates to the use of such a material as a bag or a casing for storing dry-charged batteries with a view to improving the storage properties of the battery.

Description: DRY-CHARGEDBATTERYWTIHACONTAINEROFAPOLYOLEFINMATERIAL Technical Field The present invention relates to the field of dry- charged batteries or more specifically to long-term storage of such batteries. More precisely, the invention aims at improving the storage life of the battery type having a battery container made of polyolefin material. Background of the Invention Dry charging of batteries, especially lead batte¬ ries, is performed on a large scale for long-term storage of the batteries. By this technique, it is possible to store the batteries over a long period of time after manufacture before using the batteries, primarily as starter batteries. The dry-charging technique is one of the basic conditions for long-term storage of batteries in different battery stores. Examples thereof are mili¬ tary stores, retail battery stores and other reserve stores in large organisations. The experience from long-term storage of dry-charged batteries is basically good, and the general understand¬ ing would be that this technique is a suitable way of storing batteries over long periods of time. As intimated above, many of the batteries stored in this way are starter batteries. Generally, these batteries have con¬ tainers made of polyolefins, especially polypropylene. This container material confers substantial advantages, such as good mechanical properties, low manufacturing costs and low container costs as a result of the low costs of the material employed. In other words, poly¬ olefins entail many advantages and, basically, have replaced other container materials, such as hard rubber, at least in starter batteries, which have a relatively small size. The favourable properties which polyolefins have proved to possess in these contexts have taken the focus away from the battery storage life. Obviously, existing storage properties have been considered accept¬ able in view of other favourable properties. The back- ground of the present invention however is the finding that batteries using polyolefins as container material have significantly poorer storage properties than bat¬ teries using hard rubber as container material. However, the problem is that it would probably be difficult to replace polyolefins, primarily in small-size starter bat¬ teries, with hard rubber as container material. There are many reasons for this, especially in terms of economy and performance. The cell walls are substantially thicker in batteries using hard rubber than in those using poly- olefins as container material. The thickness is about three times as large. Moreover, the hard rubber material has essentially higher density than the polyolefins. These factors entail a major difference in respect of material costs for both design alternatives. In terms of material, a container of hard rubber becomes about 7-10 times as expensive as a container of polyolefin. Batteries using containers of hard rubber do not attain the same performance based on weight and volume as bat¬ teries using containers of polyolefins, since the con- tents of the latter batteries cannot be accommodated in the former containers for the same external dimensions. This is a crucial factor when installing batteries in vehicles generally having a limited space. Like certain other current sources, lead batteries have enjoyed a development of improved performance during the 20th cen¬ tury. At the beginning of this century, use was commonly made of hard rubber material, but it now seems inconceiv¬ able to switch the development towards a performance drop. The above-mentioned facts could incite a person skilled in the art to look for alternative container materials for dry-charged batteries of the above type, but the first problem he would then encounter is that candidate materials for testing are impaired by other limitations, primarily by being harmful to the environ¬ ment and by involving large extra costs in material reco- very. PVC is one example of a material of this kind. Nor would it be possible to solve the capacity loss problem by designing the cell containers with thicker walls of the same material, i.e. polyolefin material, since, as pointed out above, there is often a limited space for mounting the battery. The investigations on which the present invention is based however also show that thicker container walls are no acceptable solution to the capacity loss problem either, which is illustrat¬ ed, for example, by the fact that an increase of the thickness of the cell container wall from 2.5 mm to 10 mm in the case of polypropylene does not yield any dramatic improvement of the capacity loss. In other words, one would again be facing the argument that the capacity drop occurring with time for dry-charged batteries using con- tainers of polyolefins must be accepted after all in view of the other favourable properties of these polyolefins. Description of the Invention The present invention represents an extremely simple and highly efficient solution to the above-mentioned problem of the capacity drop occurring in dry-charged batteries using containers of polyolefins. More specifi¬ cally, it has been found that by using a material having a higher oxygen diffusion resistance than the oxygen dif- fusion resistance of the polyolefin material of the bat¬ tery container, it is possible to achieve a substantially reduced capacity drop without having to resort to the •above-mentioned, and what is more, unsatisfactory solu¬ tion consisting in considerably increasing the wall thickness of the battery container. The new material then is preferably used as a separate bag, or a casing or the like, in which the battery is enclosed during storage. In view of the attempts reported above to increase the wall thickness of the battery container and the fact that the batteries have previously been considered suffi¬ ciently gas-insulated by existing battery containers, terminal bolt seals and seals of filling holes for elec¬ trolyte, it is surprising that the simple measure accord¬ ing to the invention has proved to produce such a drama¬ tic effect, i.e., to put it differently, that the oxygen diffusion through the container wall material has had such a great impact on the capacity drop. The present invention therefore relates, accord¬ ing to a first aspect thereof, to a dry-charged battery, especially a lead battery, having a battery container of polyolefin material, preferably polypropylene, the bat- tery being characterised in that it is provided with a surrounding bag or a surrounding casing of a material having a higher oxygen diffusion resistance than the polyolefin material of the battery container. Preferably, the bag or casing is substantially or completely enclosing the battery, which means that opti¬ mum effect is of course obtained if the bag or casing is "hermetically" sealed, so that air or oxygen cannot pene¬ trate other than through the wall of the material. If the material contains a hole or an opening somewhere, this does however not mean that the inventive concept is not complied with. The above-mentioned casing preferably also is a separate element of some type, since this confers maxi¬ mum ease of handling, but this does not exclude a solu- tion according to the invention where the battery con¬ tainer is painted or provided, by some other application, with a thinner outer layer of the material concerned having a higher oxygen diffusion resistance than the polyolefin material. The surrounding material preferably has a consider¬ ably higher oxygen diffusion resistance than the poly¬ olefin material, which means, for example, at least twice as high, more preferred at least three times as high, and the most preferred at least five times as high, an oxygen diffusion resistance. The upper limit would not be criti¬ cal, but for practical applications, an oxygen transport resistance of 3-50, especially 5-50, times as high as the oxygen transport resistance of the polyolefin material in the battery container may be considered to be especially suitable. According to yet another preferred embodiment of the battery according to the invention, the enclosing material is a material having an oxygen diffusion coef¬ ficient which is at the most 0.06 times the value of the oxygen diffusion coefficient of the polyolefin material employed in the battery container. Generally, this means that a material is selected having an oxygen diffusion coefficient which is <10~8 cm^/s. Most pre¬ ferably, this applies to the preferred material poly¬ propylene in the battery container, for which material the oxygen diffusion coefficient, according to the lite- rature, is 1.6 x IO" ' - 7 cm 2 /s. Examples of specific suitable materials for the enclosing casing or the enclosing bag are a composite or a laminate of metal foil-plastic. Another variant is to use as enclosing material a metal or a metal alloy. In the bag alternative, use is of course made of relatively thin-walled materials, which generally speaking are film or foil thicknesses. In the casing alternative, the same type of material may be used or a thicker material, e.g. a thicker box, which may be the commonest alternative in the case of metal or metal alloy. According to a second aspect of the invention, this relates to the use category, more specifically the use of •a material having a higher oxygen diffusion resistance than a polyolefin material as a bag or a casing for storage of dry-charged batteries, especially lead batte¬ ries, using battery containers of polyolefin material, with a view to improving the storage properties of the battery. The same preferred embodiments apply to this aspect of the invention in respect of oxygen transport resis- tance, oxygen diffusion coefficient and choice of mate¬ rial as apply to the first aspect of the invention de¬ scribed above, and so these preferred embodiments need not again be described in this context. Drawing The effect achieved according to the invention regarding improved storage properties is schematically illustrated in the appended drawing Figure. In this Figure, the capacity is expressed in ampere hours as a function of the storage time expressed in years. The first curve drawn as a thin full line represents the prior art being a dry-charged battery having a 2.5-mm- thick battery container of polypropylene, while the thicker full line represents an imaginary case where the wall thickness of the polypropylene container is increased to 10 mm, and the dashed line finally illu¬ strates a dry-charged battery provided according to the invention with an enclosing bag of metal foil composite material. As appears from this Figure, a dramatic reduction of the capacity drop is thus achieved with the technique according to the invention as compared with both the prior-art technique using conventional thicknesses of the battery container and the imaginary technique where the wall thickness of present-day battery containers has been increased four times. More specifically, the technique according to the invention still is at about 100% capa- •city after a storage time of seven years, whereas the capacity of the known dry-charged battery has dropped to practically 0, while the variant using the larger wall thickness has dropped to a capacity which is only about 60% of the original capacity. After storage for 10 years, the capacity with the technique according to the present invention still is above 90% of the original capacity, whereas the variant using an increased wall thickness of polypropylene has lost about 70% of its original capacity after this period of time.

Claims: CLAIMS 1. A dry-charged battery, especially lead battery, having a battery container of a polyolefin material, pre¬ ferably polypropylene, c h a r a c t e r i s e d in that it is provided with a surrounding bag or a surrounding casing, preferably substantially or completely enclosing the battery and made of a material having a higher oxygen diffusion resistance than the polyolefin material of the battery container, preferably at least twice as high, thereby exhibiting improved storage properties. 2. A battery as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the enclosing material is a mate- rial whose oxygen transport resistance is at least 3, more preferred at least 5, preferably 3-50, especially 5-50, times as high as the oxygen transport resistance of the polyolefin material of the battery container. 3. A battery as claimed in claim 1 or 2, c h a r - a c t e r i s e d in that the oxygen diffusion coeffi¬ cient of the enclosing material is l0~8 m^/s. 4. A battery as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the enclosing material is a metal foil-plastic composite or laminated material. 5. A battery as claimed in any one of claims 1-3, c h a r a c t e r i s e d in that the enclosing material is a metal, preferably in the form of a box. 6. The use of a material having a higher oxygen dif¬ fusion resistance than a polyolefin material, preferably at least twice as high, as a bag or a casing for storing dry-charged batteries, especially lead batteries, having battery containers of polyolefin material with a view to improving the storage properties of the battery. 7. Use as claimed in claim 6, c h a r a c t e r - i s e d in that the material is a material whose oxygen transport resistance is at least 3, more preferred at least 5, preferably 3-50, especially 5-50, times as high as the oxygen transport resistance of the polyolefin material of the battery container. 8. Use as claimed in claim 6 or 7, c h a r a c ¬ t e r i s e d in that the oxygen diffusion coefficient of the material is l0"8 cm^/s. 9. Use as claimed in any one of claims 6-8, c h a r a c t e r i s e d in that the material is a metal foil-plastic composite or laminated material. 10. Use as claimed in any one of claims 6-8, c h a r a c t e r i s e d in that the material is a metal, preferably in the form of a box.

Inventors: SELAANGER PER, SELAANGER, PER
Applicants: FOERSVARETS MATERIELVERK, SELAANGER PER, SELAANGER, PER
Application year: 1994
Main classification: H01M2
IPC codes: H01M2